Cosmological distance indicators

TL;DR

This review discusses three advanced cosmological distance measurement techniques—gravitational lens time delays, BAO, and HI intensity mapping—highlighting their principles, current results, and future potential to refine $H_0$ estimates.

Contribution

It provides a comprehensive overview of three key distance measurement methods beyond the local universe, emphasizing recent progress and future prospects.

Findings

Time delays from lensed quasars constrain $H_0$ with <4% uncertainty.

BAO detections up to z~2 yield $H_0$ with <2% uncertainty.

HI intensity mapping can measure BAO distances at z~0.8 with a few percent precision.

Abstract

We review three distance measurement techniques beyond the local universe: (1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and (3) HI intensity mapping. We describe the principles and theory behind each method, the ingredients needed for measuring such distances, the current observational results, and future prospects. Time delays from strongly lensed quasars currently provide constraints on $H_0$ with < 4% uncertainty, and with 1% within reach from ongoing surveys and efforts. Recent exciting discoveries of strongly lensed supernovae hold great promise for time-delay cosmography. BAO features have been detected in redshift surveys up to z <~ 0.8 with galaxies and z ~ 2 with Ly-$\alpha$ forest, providing precise distance measurements and $H_0$ with < 2% uncertainty in flat $\Lambda$CDM. Future BAO surveys will probe the distance scale with percent-level precision. HI intensity mapping has great potential to map BAO distances at z ~ 0.8 and beyond with precisions of a few percent. The next years ahead will be exciting as various cosmological probes reach 1% uncertainty in determining $H_0$, to assess the current tension in $H_0$ measurements that could indicate new physics.